Step 1: Check the materials you need.
I used an Arduino Pro Mini 3.3V. You also need USB-Serial tether to program it.
2. A battery box
I used a AA x 4 battery box with power line wires.
I used a mini breadboard by Sparkfun.
4. ECM (Electet Microphone) Module
I used an ECM module by Sparkfun.
5. RC Servo motor
I used a mini servo motor EK2-0508 by E-Sky.
6. Jumper wires
7. An Aluminum Electrolytic Capacitor
I used a 100uF capacitor.
8. A knocker hammer parts
I used a small wooden ball toy.
9. A flexible metal wire
It is for connecting the servo horn and the knocker hammer.
A steel wire 2mm in diameter will be suitable.
10. Double-stick tape
It is for making the parts bond, and for mounting the device on the door.